DeltaForce
Just going to try and consider one thing at a time; hopefully it'll actually make sense and be helpful in the end. LOL
The first obvious answer some would tell you would be "Rosalina gets better results than Meta Knight, so she's the better choice competitively.", but let's break this down more by MUs to see which character could end up supporting your Pikachu the most, especially since Meta Knight has notable positive MUs against some top tiers.
- Pikachu's most prominent struggles have been said to be Mario and Ness. Meta Knight also apparently struggles against Mario.
- Two top tier MUs that have been thought to be favourable for Meta Knight are Cloud (debated by some as being even, but it goes back and forth) and Rosalina. Neither character is considered to be a terrible problem for Pikachu.
- The MU with Mario is generally acknowledged as being in Rosalina's favour (and while Ness is definitely able to put up a fight on stage, we don't even need to discuss that MU).
- Rosalina's biggest worries are usually Cloud and Meta Knight, but again, neither one is terrible for Pikachu.
- Pikachu isn't one of Rosalina's scarier MUs, so if you had to continue using her into the next match against someone's secondary/pocket (Pocket Pikachu?!) Pikachu, you shouldn't have to worry too much. And of course, you do have your pocket Mario if you have the chance to use him.
As you mentioned yourself, one great thing about Rosalina is that she can more effectively change up her approach to a match. I'm a firm believer in every character being able to play in different ways, but some are bound by restrictions in more situations than others, so the option isn't always there as much as one would like it to be. Even without Luma, Rosalina's weird hitboxes coupled with good spacing can sometimes be an aggressive force to be reckoned with (though I'd say aggressive play in that situation is definitely not a good option as often as it can be when Luma is by her side).
When it comes to Rosalina's recovery, a lot of it comes down to timing and a mastery of the directional inputs that will get her to that ledge in just the right way (it is definitely a lot trickier to get down than Meta Knight's various recovery options, aside from dimensional cape's somewhat odd momentum and the occasional ledge bounce that can occur when using drill rush). I'm sure this is pretty obvious to you, but using Rosa's down special as a recovery timing mix-up can really help, especially if you make sure to change up how you use it, or when you just decide to completely skip it altogether. Dabuz is a really great example when it comes to using Rosa's recovery in my opinion; he rarely gets punished for actually using the recovery, or even 2-framed...
In the end with all things considered, I believe Rosalina to be the better option for you, but she is more cerebrally demanding than Meta Knight, which is something to take into consideration for a secondary, and if you find you can't get places with her, familiarity with the way a character works and dealing with various MUs can take you a lot farther than using a top tier character with great MUs who just doesn't click.
Hope this helped in some way.